Many user specifications for high performance synchronous buck mode DC-DC converters mandate the ability to monitor both current and temperature of the power elements. This is especially the case in more recently developed core voltage regulators employed by the central processing units (CPUs) of microprocessor-based electronic equipment. These regulators are typically implemented as multiphase converters having tightly specified load lines (effective output resistance), which require accurate information as to the current the regulator is supplying. Temperature measurement has customarily been limited to providing thermal compensation for the current measurement along with some form of thermal shutdown control; however, more recent specifications have been moving toward additional uses for thermal measurements, including throttling of CPU operation and system ‘telemetry’ data.
The basic half-bridge phase of a synchronous buck converter is diagrammatically illustrated in FIG. 1 as comprising a high side MOSFET 10 having its source-drain current path coupled in series with that of a low side MOSFET 20 between a pair of power supply rails (Vdd and ground (GDN)); the common connection 15 of the two MOSFETs coupled to an inductor 30 that feeds a downstream load terminal. Also shown in FIG. 1 are parasitic resistances associated with the above components, in particular the parasitic resistance RDSON10 of the high side MOSFET 10, the parasitic resistance RDSON20 of the low side MOSFET 20, and the effective series resistance ESR of the inductor 30. Now although these parasitic components allow indirect measurement of the inductor current, there are practical problems associated with their use.
First of all, all three elements have a temperature dependency, which must be measured and compensated if accurate current data is to be extracted from associated voltage measurements thereacross. Prior art systems usually employ a thermistor to measure temperature adjacent to the most important heat source. Unfortunately, cost and space considerations often limit such temperature-sensing to only a single location. In a multiphase system this becomes particularly problematic as it limits the ability to detect thermal problems that may be indicative of an impending failure. There is also the issue of manufacturing tolerance, which must either satisfy system accuracy requirements or requires calibration.
One approach to circumventing these problems is to insert a precision measurement resistor into one of the half-bridge branches, and provide temperature compensation for the precision measurement resistor. However, this results in additional energy dissipation and added components in the overall converter design. Another approach to current measurement is to incorporate a pilot (current mirror) FET with either the high side or low side MOSFET. This latter scheme is a special purpose power device and still requires accurate current measurement on the pilot current.